1. Field of the Invention
The present invention relates to an image recording apparatus, an image recording method, and a data generation apparatus.
2. Description of the Related Art
There are known image recording apparatuses that record an image by repeatedly performing recording scanning and sub-scanning. In the recording scanning, the image forming apparatuses discharge ink while relatively moving a discharge head, on which a plurality of recording nozzles configured to discharge the ink is arranged, relative to a unit region on a recording medium. In the sub-scanning, the image forming apparatuses convey the recording medium in a sub-scanning direction, which is a direction intersecting with a recording scanning direction. As a recording method employed by theses image forming apparatuses, there is known a recording method for forming an image by performing recording scanning a plurality of times on a unit region with use of data generated by dividing data of an image to be formed on the unit region according to a mask pattern in which recording permitting pixels permitted to record dots during each scanning are arranged.
In recent years, in the field of inkjet recording, users have been demanding a higher image quality for a recorded image, and have been demanding a recorded image with a low grainy effect. In addition to these demands, the users have been demanding a recording process capable of fixing a color material in ink onto a recording medium with a poor chemical affinity for the ink to deal with an expansion in the range of types of used inks and recording media.
As an example of a method for reducing the degree or frequency of beading between applied ink droplets at the recording apparatuses that record an image by performing scanning a plurality of times on a unit region on a recording medium, Japanese Patent Application Laid-Open No. 2006-44258 discusses the following technique. According to this technique, an image recording apparatus determines a layout of recording permitting pixels in a mask pattern with use of repulsive potentials provided to pixels with dots arranged thereon, and dispersively arranges dots so as to avoid beading between ink droplets applied on a recording medium at an intermediate stage during image formation.
On the other hand, Japanese Patent Application Laid-Open No. 1-113249 discusses a method for fixing a color material contained in ink onto a recording medium by applying heat to the ink with use of a heating unit when the ink is applied onto the recording medium. This method promotes fixation of the color material by promptly evaporating moisture contained in the ink.
However, as a result of consideration, the inventors have discovered the following problem; if ink is discharged onto a recording medium maintained at a relatively high temperature, density unevenness and a visually noticeable grainy effect occur in an image formed on a certain region on the recording medium depending on the type of the recording medium.
In the following description, this problem will be described based on an example of an image recording apparatus configured to heat a recording medium.
FIG. 1 is a schematic view illustrating how ink droplets act when the ink droplets are applied onto a heated recording medium, at the time of recording using an image forming apparatus provided with a heating unit.
As illustrated in part (a) in FIG. 1, ink is discharged from a discharge head that scans a recording medium in a main scanning direction indicated by an arrow in FIG. 1. The recording medium is heated by a heater from a position opposite the recording medium. The recording medium with the ink applied thereon is conveyed in the sub-scanning direction indicated by an arrow in FIG. 1.
Part (b) in FIG. 1 illustrates a change in heat distribution on the recording medium between before and after the application of the ink, indicating the changing state as viewed in cross-section perpendicular to the recording medium along the sub-scanning direction illustrated in part (a) in FIG. 1. With the passing of time from a state b1, the state shifts to states b2, b3, b4, and b5. Immediately before the application of the ink (the state b1), a heated region (indicated by white arrows) is wider than a region of the recording medium where an image is formed by one scanning operation by the discharge head (a scanning region (a region C in part (b) in FIG. 1)). Therefore, the recording medium has high-temperature and even heat distribution over a range wider than one scanning region. For example, if the recording medium is made from, for example, a vinyl compound, the heat conductivity should be high, whereby it is considered that the recording medium has high-temperature and even heat distribution over a wide range.
It should be noted that not only the heat directly supplied from the heating unit but also heat supplied from the heated recording medium are used for evaporation of moisture in the ink when the ink is applied onto the one scanning region of the recording medium. Therefore, during a short period immediately after the application of the ink, the temperature reduces at portions of the recording medium where the ink 27 is applied (the state b2).
On the other hand, because moisture is not evaporated on a region of the recording medium outside the one scanning region C (a non-image forming region), this region is not subject to such a reduction in the stored heat.
Therefore, regarding an end region (A) of the one scanning region in the sub-scanning direction, the non-image forming region exists near the region (A), whereby the heat stored in the non-image forming region is transmitted to the region (A) via the recording medium, and, therefore, the lost heat can be relatively easily compensated for (the state b3).
However, regarding a central portion of the one scanning region in the sub-scanning direction, there is no non-image forming region near the central portion, whereby a heat recovery is relatively slow here compared to the end portion.
As a result, during a period of a certain time after the application of the ink, such as a period from the state b3 to the state b4, the recording medium has such temperature unevenness within the one scanning region thereof that the temperature at the central portion is lower than the temperature at the end portion in the sub-scanning direction.
If the temperature unevenness occurs, it takes a longer time to evaporate moisture in the ink at the low-temperature portion, increasing a time to fix the ink onto the recording medium compared to the high-temperature portion. Therefore, the low-temperature portion has a larger number of opportunities for a plurality of applied ink droplets to move and then contact one another before the fixation compared to the high-temperature portion, and is subject to easy displacement of the ink droplets when they contact one another compared to the high-temperature portion. This situation facilitates occurrence of beading, leading to a possibility of development of density unevenness and deterioration of a grainy effect in a recorded image.
One possible method against this problem is to supply a large heat amount from the heating unit to eliminate the above-described low-temperature portion from the recording medium, but this method may result in an increase in the size of the heating unit and complexity of the mechanism of the recording apparatus.
As described above, the inventors of the present invention have discovered the new problem of an image quality being affected by temperature distribution of a recording medium when ink droplets are applied onto the recording medium.